At least one person takes their life every day while suffering from a chronic or terminal illness, and the government is neglecting this hidden trend, the thinktank Demos has said.

In a study, The Truth About Suicide, researchers found at least 10% of suicides in Britain are linked to terminal or chronic illness, accounting for more than 400 deaths a year. The thinktank, which said the study marked the first attempt to estimate the scale of suicides related to illness, wanted to challenge the notion that taking one's own life is largely about a patient's mental health rather than physical state. Researchers also found some people were killing themselves at a "younger age in order to avoid severe symptoms and greater pain later in life".

The figures come from a mixture of sources, including data from freedom of information requests to 147 primary care trusts, which are supposed to conduct annual suicide audits.

Researchers also conducted a series of interviews with serving and recently retired coroners. Demos also had access to suicide inquest files in Norwich from May 2006 to December 2010 to identify the proportion of suicides that involved people with terminal or chronic health conditions. It said that of the 4,390 individual suicide cases last year, 10% concerned people "experiencing some form of serious physical illness as an influencing factor".

The researchers said patients with such conditions "should be considered a high- risk group for suicide within national policy, and much greater attention should be given to providing better medical, practical and psychological support". The issue has become a fixture in public debate as growing numbers of UK citizens with chronic or terminal conditions have travelled to the assisted-death organisation Dignitas in Switzerland to be helped to end their lives. Last year, a coroner recorded a verdict of suicide over the death of Michelle Broad, wife of the former England cricketer and international referee Chris Broad, who had motor neurone disease.

Louise Bazalgette, author of the report, said the "lack of attention paid to people with terminal or chronic illness committing suicide is a gross dereliction of duty on the part of the government and health services. The difficulty we experienced in tracking down evidence of the relationship between physical illness and suicide suggests a wilful avoidance of what is an extremely important public health issue.

"The results are devastating: at least 400 people with terminal or chronic illness commit suicide every year and this cannot continue to be ignored."

She said a government consultation on suicide prevention, issued last month, focused on the same "at risk" groups identified by Labour in 2002: those using mental health services, prisoners and people with a history of self-harm.

"Eighty prisoners committed suicide last year. That is only a fifth of suicides of patients with chronic or terminal conditions. There's an urgent need for support," she said.

Experts agreed hospital doctors and GPs could do more to identify patients at risk from suicide but warned that the issue was "more complicated".

Linda Gask, professor of primary care psychiatry at the University of Manchester, said it was wrong to think "being depressed enough to commit suicide is either because of your mental health or your physical health: one is linked to another. We know that 30% of diabetes patients are depressed. Not all of them commit suicide.

"On the other hand, those travelling to Dignitas would say they are not depressed. They argue they are making a rational decision because they do not want to live like this."

Charities were also wary of clearly ascribing the cause of a suicide to either physical or mental health problems.

Jo Ferns, director of research for the Samaritans, said the issue was one of "risk" and what factors increased the chance of suicide.

"People with chronic conditions could be not suicidal but perhaps apathetic about living or dying. They are not saying they will kill themselves but taking decisions that do increase the risks. I think that's what we have to watch for."

A Department of Health spokesman said its mental health strategy recognised "that physical illness increases the risk of mental health problems and vice versa. We are consulting with bereaved families and experts in general practice, local government, and other organisations on a new strategy to prevent suicides.

"The consultation calls upon healthcare professionals to be alert to mental health issues, especially depression, in the patients that they see for known physical health problems, and to take the right steps to help people with long-term conditions have a better quality of life."